Visible radiation type photocatalyst and production method thereof

ABSTRACT

The invention is directed to a catalyst having activity under the irradiation of a visible light, the catalyst being an oxide semiconductor such as an anatase type titanium dioxide, having stable oxygen defects. A method for producing a catalyst having activity under the irradiation of a visible light which comprises treating an oxide semiconductor with hydrogen plasma or with a plasma of a rare gas element, comprising performing the treatment in a state substantially free from the intrusion of air into the treatment system is also provided. An article comprising a base material having the catalyst above provided on the surface thereof and a method for decomposing a substance, comprising bringing an object to be decomposed into contact with the catalyst above under the irradiation of a light containing at least a visible radiation are disclosed. A novel photocatalyst which enables use of a visible radiation is provided, as well as a method utilizing the photocatalyst for removing various substances containing an organic matter or bacteria by photodecomposition.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photocatalyst having a visible lightactivity, to a method for producing the photocatalyst, a method ofphotodecomposition using a light involving visible rays, and to a deviceusing the photocatalyst.

2. Background Art

Various studies on deodorization and sterilization using photocatalystshave been made to present, and some of them have been put to practice.For instance, in WO94/11092 is disclosed a method for treating air usinga photocatalyst under the irradiation of room light. In Japanese PatentLaid-Open No. 102678/1995 is disclosed a method for preventing medicalinfection from occurring in hospitals by using photocatalysts. In bothcases, an oxide semiconductor such as titanium dioxide and the like isused as the photocatalyst, which requires an ultraviolet radiation 400nm or shorter in wavelength for the excitation.

However, sunlight or an artificial light source used as the excitationlight source also includes visible rays in addition to ultravioletradiations. Yet, the visible rays are not used in the photocatalystscomprising oxide semiconductors such as titanium dioxide as describedabove; hence, such photocatalysts are extremely in efficient as viewedfrom the point of energy conversion efficiency.

It is well known that titanium dioxide acquires photocatalytic activityby injecting metallic ions such as chromium by using ion implantationmethod. However, this method is practically unfeasible because itinvolves the use of voluminous equipment.

On the other hand, it is reported that the catalytic activity oftitanium dioxide under ultraviolet radiation can be increased byproviding it with TiC coating using plasma CVD (see Japanese PatentLaid-Open No. 87857/1997). However, the literature does not teach anyphotocatalytic activity under the irradiation of visible light.

In the light of such circumstances, an object of the present inventionis to provide a novel photocatalyst capable of using visible light.

A second object of the present invention is to provide a productionmethod for the photocatalyst above.

Furthermore, a third object of the present invention is to provide amethod for removing various types of substances including organic matteror bacteria by photode composition using the novel photocatalystdescribed above.

Further, a fourth object of the present invention is to provide a deviceusing the novel photocatalyst above.

SUMMARY OF THE INVENTION

The present invention relates to a catalyst that exhibits activity underthe irradiation of visible light, characterized by that it is an oxidesemiconductor having stable oxygen defects.

As the oxide semiconductor, there can be mentioned, in addition totitanium dioxide, hafnium oxide, zirconium oxide, strontium titanate, atitanium oxide-zirconium oxide based complex oxide, a siliconoxide-titanium oxide based complex oxide, etc.

As the catalyst above, there can be mentioned, for instance, a catalysthaving activity under the irradiation of visible light and which is ananatase type titanium dioxide having stable oxygen defects.

The present invention furthermore refers to a method for producing avisible-radiation activating type photocatalyst, comprising hydrogenplasma treating or rare gas element plasma treating an oxidesemiconductor, characterized by that the treatment is performed in atreatment system under a state substantially free from the intrusion ofair. In addition, the present invention relates to a method forproducing a visible-radiation activating type photocatalystcharacterized by that rare gas element ions are injected to at least apart of the surface of the oxide semiconductor. Further, the presentinvention relates to a method for producing a catalyst exhibitingactivity under the irradiation of visible radiation, characterized bythat the oxide semiconductor is heated in vacuum. In particular, theoxide semiconductor above can be an anatase type titanium dioxide.Furthermore, the present invention relates to a catalyst produced by theproduction method above according to an aspect of the present invention,which exhibits activity under the irradiation of visible radiation, andmentioned as examples of said oxide semiconductor are, for instance,titanium dioxide, hafnium oxide, zirconium oxide, strontium titanate, atitanium oxide-zirconium oxide based complex oxide, a siliconoxide-titanium oxide based complex oxide, etc.

Further, the present invention relates to an article characterized bythat the catalyst according to the present invention above is providedon the surface of a base material.

In addition, the present invention relates to a method for photodecomposing a substance, comprising, under the irradiation of a lightincluding visible rays, bringing the catalyst according to the presentinvention above or the article according to the present invention intocontact with a medium containing the object to be decomposed, therebyeffecting the decomposition of the object to be decomposed.

Further, the present invention refers to a device for use inphotodecomposition, comprising a photocatalyst unit having the catalystaccording to the present invention provided on the surface of a basematerial and a light source for irradiating a light including visibleradiation to the photocatalyst above.

The present invention refers, furthermore, to an electrode for use insolar cells and to an electrode for use in the photodecomposition ofwater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the X-ray diffractograms for each of thesamples obtained before and after plasma treatment;

FIG. 2 is a diagram showing the ESR spectrum of a specimen (anatase-typetitanium dioxide) before plasma treatment; and

FIG. 3 is a diagram showing the ESR spectrum of a catalyst according tothe present invention (a specimen (anatase-type titanium dioxide) aftersubjecting it to plasma treatment)

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The catalyst according to the present invention is characterized by thatit is an oxide semiconductor having stable oxygen defects. Furthermore,the catalyst according to the present invention is a catalyst thatexhibits activity under the irradiation of a visible light. As the oxidesemiconductor above, there can be mentioned, for instance, titaniumdioxide, hafnium oxide, zirconium oxide, strontium titanate, a titaniumoxide-zirconium oxide based complex oxide, a silicon oxide-titaniumoxide based complex oxide, etc., but is not only limited thereto. Theoxide semiconductor may be a rutile type titanium dioxide or an anatasetype titanium dioxide. Particularly preferred as the oxide semiconductorfrom the viewpoint of practice is an anatase type titanium dioxide.

The case of anatase type titanium dioxide, which is a representativeoxide semiconductor, is described below. The catalyst according to anembodiment of the present invention is characterized by that it is ananatase type titanium dioxide having stable oxygen defects, and that itexhibits activity under the irradiation of a visible light.

Further, the catalyst according to the present invention maybe atitanium dioxide which yields a diffractogram obtainable by X-raydiffraction (XRD) substantially free from patterns other than that ofanatase type titanium dioxide.

The degree of oxygen defects of the anatase type titanium dioxide thatis used as the catalyst according to the present invention can bespecified by the ratio of peak area obtained by X-ray photoelectronspectroscopy assigned to the 1s electrons of oxygen participating in thebonds with titanium to that assigned to the 2p electrons of titanium(O1s/Ti2p), and the value is, for instance, 1.99 or lower. A morepreferred peak area ratio (O1s/Ti2p) is in a range of from 1.5 to 1.95.The stability of the oxygen defects of the oxide semiconductorsignifies, in case the catalyst according to the present invention is,for instance, an anatase type titanium dioxide having oxygen defects,that the area ratio (O1s/Ti2p) described above is maintainedsubstantially constant even in case the catalyst is left in air for 1week or longer. It is well known that oxygen defects generate whentitanium dioxide is reduced by gaseous hydrogen, however, the oxygendefects that are obtained by reduction using gaseous hydrogen areextremely unstable, and such defects disappear in a short period whenleft in air. On the other hand, the oxygen defects that are present inthe catalyst according to the present invention are extremely stable,such that they remain stable for at least half a year even when thecatalyst is left under ambient. Furthermore, when the catalyst accordingto the present invention is used in a photocatalytic reaction, theoxygen defects above do not diminish in a short period of time, but itcan be used stably as a catalyst.

Concerning the band gap of titanium dioxide, an anatase type yields avalue of 3.2 eV, and a rutile type yields 3.0 eV. Both types areactivated solely by ultraviolet radiation, but the catalyst according tothe present invention not only exhibits photoactivity under theirradiation of an ultraviolet radiation inherent to titanium dioxide,but also is photoactivated by visible light alone. The degree ofphotoactivation of the catalyst according to the present inventionobtainable by the irradiation of a visible light depends on the quantityof the oxygen defects and the like; however, in case of an anatase typetitanium dioxide, for instance, if the activity obtainable under theirradiation of a black light radiation with radiations 400 nm or longerin wavelength being cut off is taken as 100, an activity of at least 5can be obtained under an irradiation of light radiated from a halogenlamp with radiations 420 nm or shorter in wavelength being cut off, andin general, an activity of 20 or higher is achieved. Furthermore, theactivity of the catalyst according to the present invention under theirradiation of a visible light is attributed to the oxidation activityor the reduction activity inherent to anatase type titanium dioxide.

The activity of the catalyst according to the present invention underthe irradiation of a visible light signifies that the catalyst at leastexhibits an NOx oxidation activity under the irradiation of visibleradiations 400 to 600 nm in wavelength. The titanium oxide knownheretofore exhibits activity of a certain degree under the irradiationof a visible light having a wavelength in the vicinity of 400 nmattributed to the band gap described above. However, no catalysts havinga photocatalytic activity for visible radiations in the wavelengthregion of longer than 500 nm up to a vicinity of 600 nm are known topresent.

For instance, the catalyst according to the present invention obtainableby the hydrogen plasma treatment or by the rare gas element plasmatreatment yields a NOx oxidation activity (NO removal activity) of atleast 30 under the irradiation of a light 460 nm in wavelength,preferably 50 or higher, and most preferably, 60 or higher, with respectto the NOx oxidation activity (No removal activity) of 100 achievedunder the irradiation of a light 360 nm in wavelength. Further, the NOxoxidation activity (NO removal activity) obtained under the irradiationof a light 560 nm in wavelength is at least 5, preferably 10 or higher,and most preferably, 15 or higher.

In case of an anatase type titanium oxide manufactured by IshiharaSangyo Kaisha which is reputed to have a high photocatalytic activity,the NOx oxidation activity (NO removal activity) obtained under theirradiation of a light 460 nm in wavelength is approximately null, andcompletely no activity is obtained under the irradiation of a light 560nm in wavelength, with respect to the NOx oxidation activity (NO removalactivity) of 100 obtained under the irradiation of a light 360 nm inwavelength.

For the measurement of the NOx oxidation activity (NO removal activity)as referred above, a 300-W xenon lamp was used for the light source, anda monochromatic light having a half band width of 20 nm was irradiatedby using an irradiation apparatus manufactured by JASCO. Morespecifically, the radiations having a wavelength of 360 nm, 460 nm, and560 nm are each a monochromatic light having a half band width of 20 nm.

The catalyst having a photocatalytic activity with respect to a visiblelight in the wavelength region up to the vicinity of 600 nm inwavelength is, for instance, a titanium oxide having stable oxygendefects, which yields a signal having a g value of from 2.003 to 2.004in the ESR measured in darkness at 77K under vacuum, provided that theyyield a signal higher in intensity than the g value of from 2.003 to2.004 above when measured at least under the irradiation of light in thewavelength region of from 420 to 600 nm at 77K in vacuum. The signalshaving a g value of from 2.003 to 2.004 measured by ESR under theconditions above are known to be assigned to the oxygen defects oftitanium oxide. However, it is not known that a photocatalyst havingexcellent photoactivity with respect to visible radiations can beprovided in case the above signals measured under the irradiation oflight in the wavelength region of from 420 to 600 nm at 77K in vacuum isgreater in intensity as compared to those obtained in darkness as above.

Preferably, the ratio of the intensity I0 of the ESR signal having ameasured g value in the range of from 2.003 to 2.004 under darkness at77K in vacuum to the intensity IL of the ESR signal having a measured gvalue in the range of from 2.003 to 2.004 at 77K in vacuum under theirradiation of a radiation in the wavelength region of at least 420 to600 nm, IL/I0, is over 1; more preferably, the ratio (IL/I0) is 1.3 orhigher, and most preferably, it is 1.5 or higher.

In addition to above, preferred from the viewpoint of obtaining aphotocatalyst having a higher activity with respect to visibleradiations is that a signal assigned to Ti³⁺, which yields a g value of1.96 when measured by ESR in darkness at 77K in vacuum, is substantiallynot observed.

The same applies to oxide semiconductors other than titanium dioxide.Hence, such an oxide semiconductor is photoactivated not only under theirradiation of an ultraviolet radiation, but by the irradiation of avisible radiation alone. The degree of photoactivation by theirradiation of a visible light depends on the quantity of oxygen defectsand the like. The activity of the catalyst according to the presentinvention under the irradiation of a visible light is attributed to theoxidation activity or the reduction activity inherent to the oxidesemiconductor.

Furthermore, the activity of the catalyst under the irradiation of avisible light according to the present invention is a decompositionactivity for inorganic and organic substances, or a bactericidalactivity.

There is no particular limitation concerning the shape of the catalystaccording to the present invention, and can be used in a granular, athin-film like, or a sheet-like shape. However, the shape is not limitedthereto. The granular oxide semiconductor (catalyst) may be finely sizereduced with an aim to increase the activity, or maybe pelletized toease the handling thereof. Concerning the surface of the oxidesemiconductor (catalyst) shaped into thin films or sheets, it may beroughened to increase the activity. Furthermore, other components may beadded to the titanium dioxide above at such a degree that their additiondoes not impair the activity to visible light of the catalyst accordingto the present invention.

The catalyst according to the present invention can be obtained, forinstance, by a method comprising treating an oxide semiconductor withhydrogen plasma or a plasma of a rare gas element, and the method ischaracterized by that the treatment is performed in a statesubstantially free from the intrusion of air into the treatment system.

The oxide semiconductor above may be, for instance, titanium dioxide,hafnium oxide, zirconium oxide, strontium titanate, a titaniumoxide-zirconium oxide based complex oxide, a silicon oxide-titaniumoxide based complex oxide, etc.

The anatase type titanium dioxide that is used as the starting materialmay be obtained by a wet method; for instance, there may be used atitanium dioxide produced by sulfuric acid method, or a titanium dioxideprepared by a dry method.

The treatment using hydrogen plasma can be performed by generatinghydrogen plasma obtained by introducing gaseous hydrogen to an oxidesemiconductor provided under a reduced pressure state while irradiatingan electromagnetic wave, for instance, a microwave or a radio wave, andexposing the oxygen semiconductor to the plasma for a predeterminedduration of time. In the treatment using a plasma of a rare gas element,the treatment can be carried out by generating a plasma of a rare gaselement obtained by introducing gaseous rare gas element to an oxidesemiconductor provided under a reduced pressure state while irradiatingan electromagnetic wave, for instance, a microwave or a radio wave, andexposing the oxygen semiconductor to the plasma for a predeterminedduration of time. As the rare gas element, there can be mentioned, forinstance, helium, neon, argon, krypton, xenon, radon, etc., however,from the viewpoint of ease in availability, preferred are the use ofhelium, neon, argon, etc.

The reduced pressure state is, for instance, 10 Torr or lower, or may be2 Torr or lower. The output of the electromagnetic wave can be properlyselected by taking the quantity of the oxide semiconductor to be treatedor the state of plasma generation into consideration. The amount ofgaseous hydrogen or the gaseous rare gas element to be introduced intothe system can be properly selected by taking the reduced pressure stateor the state of plasma generation into consideration. The time durationof exposing the oxide semiconductor to the hydrogen plasma or the plasmaof the rare gas element is properly selected depending on the quantityof oxygen defects that are introduced into the oxide semiconductor.

The production method according to the present invention ischaracterized by that it is performed Ian state substantially free fromthe intrusion of air into the plasma treatment system. The state“substantially free from the intrusion of air into the plasma treatmentsystem” signifies that the vacuum degree inside the tightly sealedsystem takes at least 10 minutes to make a change of 1 Torr. The lessthe intrusion of air occurs, the more easily the oxygen defects areintroduced into the oxide semiconductor.

Furthermore, if desired, the hydrogen plasma above may contain gasesother than gaseous hydrogen. As gases other than gaseous hydrogen, therecan be mentioned, for instance, those of rare gas elements. In theproduction method according to the present invention, oxygen defects canbe introduced into the oxide semiconductor by using plasma of hydrogenor of rare gas element, and the coexistence of the rare gas elementtogether with hydrogen plasma is not essentially required for theintroduction of oxygen defects. The same can be said to the case ofusing the plasma of rare gas element. Thus, the plasma of a rare gaselement may contain gases other than those of rare gas elements ifdesired, and as such gases, there can be mentioned gaseous hydrogen.However, the coexistence of hydrogen in the plasma of rare gas elementis not essential in introducing oxygen defects.

The catalyst according to the present invention can be also produced bya method comprising ion-injecting the ions of a rare gas element to atleast a part of the surface of the oxide semiconductor. The ionimplantation method can be carried out by using the methods andapparatuses currently used in the semiconductor industry. The conditionsof ion implantation can be properly determined depending on the quantityof the ions of the rare gas element, the type of the oxidesemiconductor, etc. Further, as the rare gas elements, there can bementioned, for instance, helium, neon, argon, krypton, xenon, radon,etc., however, from the viewpoint of ease in availability, preferred arethe use of helium, neon, argon, etc.

Furthermore, the production of the catalyst according to the presentinvention is not only limited to powder, but there can also be usedtitanium oxide fixed to a substrate by using a proper,binder, etc.

The catalyst according to the present invention may be produced by amethod comprising heating an oxide semiconductor in vacuum. Forinstance, by subjecting titanium dioxide to a heat treatment in highvacuum, or by subjecting it to hydrogen reduction while heating underhigh vacuum, it is known that oxygen defects are generated to causevisible light absorption. However, it is not known that the resultingtitanium dioxide containing oxygen defects function as catalystsexhibiting activity under the irradiation of a visible light Theproduction method above maybe, for instance, a method comprising heatingan anatase type titanium dioxide to a temperature of 400° C. or higherunder a vacuum of 1 Torr or lower. The time duration of the treatmentmay be properly set depending on the vacuum degree and the temperature,but in case of a treatment at 400° C. under a vacuum of 0.1 Torr orlower, the time duration may be set in a range of from 30 minutes to 1hour.

As described above, an anatase type titanium dioxide subjected to ahydrogen plasma treatment or a rare gas element plasma treatment, orsuch injected with ions contains stable oxygen defects, and become acatalyst exhibiting activity under the irradiation of a visible light.Similarly, a rutile type titanium dioxide, zirconium oxide, hafniumoxide, strontium titanate, etc., also can provide a catalyst exhibitingactivity under the irradiation of a visible light by subjecting them toa hydrogen plasma treatment or a rare gas element plasma treatment, orby injecting ions thereto. However, the intensity of activity and thewavelength dependence of the activity under the irradiation of a visiblelight greatly differ depending on the type of the oxide semiconductorand the treatment method. zirconium oxide is a semiconductor, however,it has a large band gap, and was thereby believed to show no function asa practically useful photocatalyst. Still, however, by subjecting it toa hydrogen plasma treatment or a rare gas element plasma treatment, orby injecting ions thereto in accordance with the production method ofthe present invention, it was found to provide a catalyst exhibitingactivity under the irradiation of UVa or a visible radiation.

As a result of surface analysis using ESCA, a zirconium oxide subjectedto a hydrogen plasma treatment or a treatment using a rare gas elementplasma, or such injected with ions was observed to generate a tracequantity of zirconium carbide and oxygen defects. A rutile type titaniumdioxide functions as a photocatalyst under the irradiation of anultraviolet radiation, however, it was not practically used as aphotocatalyst because it is inferior in activity as compared with ananatase type counterpart. However, it has been found that the rutiletype titanium dioxide can be used as a catalyst having activity underthe irradiation of a visible light by treating it with hydrogen plasmaor plasma of a rare gas element, or by treating it by ion implantationin accordance with the production methods of the present invention.Conventionally, no activity had been found on hafnium oxide andstrontium titanate under the irradiation of a visible light; however,activity under the irradiation of a visible light was confirmed in thecase of the catalyst having stable oxygen defects according to thepresent invention.

The present invention furthermore relates to the catalyst according tothe present invention as described above, or to an article havingprovided with the catalyst produced by the production method accordingto the present invention on the surface of a base material. As the basematerial, for instance, there can be used an exterior wall of abuilding, an exterior plane of a roof or a ceiling, an outer plane or aninner plane of a window glass, an interior wall of a room, a floor or aceiling, a blind, a curtain, a protective wall of highway roads, aninner wall inside a tunnel, an outer plane or a reflective plane of anilluminating light, an interior surface of a vehicle, a plane of amirror, etc.

The catalyst can be provided to the base material by, for instance,coating or spraying the catalyst according to the present invention or apaint containing the particles of the catalyst produced by theproduction method according to the present invention. Furthermore, abase material having a layer of an oxide semiconductor such as titaniumdioxide may be subjected to a hydrogen plasma treatment in accordancewith the production method according to the present invention to providethe surface of the oxide semiconductor layer as the catalyst accordingto the present invention to thereby obtain the article according to thepresent invention.

Further, the method for photo decomposing a substance according to thepresent invention comprises bringing a medium containing the object tobe decomposed into contact with the catalyst according to the presentinvention, a catalyst produced by the production method according to thepresent invention, or an article according to the present inventionunder the irradiation of a light containing a visible light, so that thedesired object to be decomposed may thereby decomposed.

The object to be decomposed may be at least one type of substanceselected from the group consisting of inorganic compounds, organiccompounds, microorganisms, and tumor cells. The medium may be, forinstance, water or air. More specifically mentioned as such a medium isair having bad odor or a harmful substance (e.g., nitrogen oxides,formalin, etc.), organic matters (e.g., waste water containing crude oilor petroleum products, marine water, etc.). The light containing visiblelight may be a solar radiation or an artificial light. An artificiallight source relates to any type of a light source capable of supplyinga light containing visible light, such as the rays irradiated from afluorescent lamp, an incandescent lamp, a halogen lamp, etc.

Further, the photodecomposition device according to the presentinvention comprises a photocatalyst unit having the catalyst accordingto the present invention or the catalyst produced by the productionmethod according to the present invention provided on the surface ofabase material, and a light source for irradiating a light containing alight containing a visible light to the aforementioned photocatalyst.The photocatalyst unit can be, for instance, a filter of an air cleaner.As the light source for irradiating a visible light, there can bementioned, for instance, a fluorescent lamp, an incandescent lamp, ahalogen lamp, etc.

In case an air contains a substance that is the source of bad odor, byusing the method or the device according to the present invention and bybringing an air containing the object to be decomposed into contact withthe photocatalyst or the photocatalyst unit (article) under theirradiation of a light containing at least a visible radiation, thesource substance of the bad odor contained in the air can be decomposedby bringing it into contact with the catalyst to thereby reduce orremove the bad odor. In case the air contains bacteria, at least a partof the bacteria contained in air can be destroyed by bringing the airinto contact with the catalyst. If the air contains both bad odor andbacteria, it can be readily understood that the aforementioned reactionsoccur in parallel with each other.

By using the method or the device according to the present invention andby bringing water containing the object to be decomposed into contactwith the photocatalyst or the unit (article) using the photocatalystaccording to the present invention under the irradiation of a light atleast containing a visible radiation, and in case the water contains anorganic matter, the organic matter contained in the water can bedecomposed by its contact with the catalyst. If the water containsbacteria, the bacteria present in the water can be destroyed by bringingthe water into contact with the catalyst. If the water contains bothorganic matters and bacteria, it can be readily understood that theaforementioned reactions occur in parallel with each other.

Furthermore, the electrode for solar cells and the electrode forphotodecomposition of water according to the present invention comprisea material based on an oxide semiconductor, such as an anatase typetitanium dioxide, containing stable oxygen defects, and the details ofthe materials and the production methods are as described above.Further, the electrode for solar cells and the electrode forphotodecomposition of water according to the present invention comprisea catalyst made of an oxide semiconductor subjected to a treatmentaccording to the production method of the present invention. In case ofapplying the present invention to a solar cell electrode, a solar cellcan be assembled by using a known system while taking thecharacteristics of the present electrode into consideration. In case ofapplying the present invention to an electrode for photo-decomposingwater, the photodecomposition of water can be performed by using a knownmethod and device.

EXAMPLES

The present invention is described in further detail by making referenceto the non-limiting examples as follows.

Example 1

A 10-g portion of a powder (60 mesh or less in granularity) of ananatase type titanium dioxide was placed inside a 200-ml volume quartzreaction tube. The quartz reaction tube was connected to a plasmagenerating apparatus, and after evacuating the inside of the system byusing a vacuum pump, a 400-W power electromagnetic wave (at a frequencyof 2.45 GHz) was irradiated to the powder of anatase type titaniumdioxide placed inside the reaction tube to thereby generate a plasma byusing a Tesla coil. Then, gaseous H₂ was introduced inside the system ata flow rate of 30 ml/min to set the pressure inside the system to about1 Torr. The treatment was continued for 30 minutes while stirring theanatase type titanium dioxide powder placed inside the reaction tube.

Time duration of 40 minutes was necessary to increase the vacuum degreeinside the plasma treatment system for 1 Torr without introducing a gasand while cutting off the pump evacuation.

The anatase type titanium dioxide powder thus obtained was subjected toX-ray photoelectron spectroscopy (XPS), and the area of the peaksassigned to the 2p-electron of titanium (458.8 eV (Ti2p_(3/2)) and 464.6eV (Ti2p_(1/2))) as well as that of the peak assigned to the 1s-electronof the oxygen bonded to titanium (531.7 eV (O1s)) was obtained. The arearatio (O1s/Ti2p) was found to be 1.91. The area ratio (O1s/Ti2p) of ananatase type titanium dioxide powder not subjected to plasma treatmentwas 2.00.

The sample was left to stand in air for 1 week, and the area ratio wasobtained by performing the same measurement to obtain an area ratio(O1s/Ti2p) of 1.91. The sample was further left for 1 month, but nochange was found on the area ratio (O1s/Ti2p).

The samples before and after the plasma treatment above were eachsubjected to X-ray diffraction analysis, and, as a result, no change wasobserved between the anatase type titanium dioxide samples before andafter the plasma treatment.

Example 2

A 10-g portion of a powder (60 mesh or less in granularity) of ananatase type titanium dioxide was placed inside a 200-ml volume quartzreaction tube. The quartz reaction tube was connected to a plasmagenerating apparatus, and after evacuating the inside of the system byusing a vacuum pump, a 400-W power electromagnetic wave (at a frequencyof 2.45 GHz) was irradiated to the powder of anatase type titaniumdioxide placed inside the reaction tube to thereby generate a plasma byusing a Tesla coil. Then, gaseous argon was introduced inside the systemat a flow rate of 10 ml/min to set the pressure inside the system toabout 1 Torr. The treatment was continued for 120 minutes while stirringthe anatase type titanium dioxide powder placed inside the reactiontube.

Time duration of 40 minutes was necessary to increase the vacuum degreeinside the plasma treatment system for 1 Torr without introducing a gasand while cutting off the pump evacuation. The anatase type titaniumdioxide powder thus obtained was subjected to X-ray photoelectronspectroscopy (XPS), and the area of the peaks assigned to the2p-electron of titanium (459.5 eV (Ti2p_(3/2)) and 465.4 eV(Ti2p_(1/2))) as well as that of the peak assigned to the 1s-electron ofthe oxygen bonded to titanium (530.0 eV (O1s)) was obtained. The arearatio (O1s/Ti2p) was found to be 1.89. The area ratio (O1s/Ti2p) of ananatase type titanium dioxide powder not subjected to plasma treatmentwas 2.00.

The sample was left to stand in air for 1 week, and the area ratio wasobtained by performing the same measurement to obtain an area ratio(O1s/Ti2p) of 1.89. The sample was further left for 1 month, but nochange was found on the area ratio (O1s/Ti2p).

The samples before and after the plasma treatment above were eachsubjected to X-ray diffraction analysis, and, as a result, no change wasobserved between the anatase type titanium dioxide samples obtainedbefore and after the plasma treatment.

Example 3

A production method of a catalyst according to the present inventioncomprising injecting ions of rare gas elements to the surface of anoxide semiconductor, an anatase type titanium dioxide, is describedbelow.

As the equipment was used a medium current ion implantation apparatus,ULVAC IKX-7000 manufactured by ULVAC Co., Ltd.

The method comprises, after introducing gaseous argon, irradiatingelectron beam to the sample for ionization, subjecting the ionizedspecies to mass spectroscopy to separate and take out argon ions, andthe argon ions were accelerated in an accelerator (at a direct currentvoltage of 100 kV) to inject argon ions to the target.

As the target, a glass plate 6 cm in diameter (which is about 0.2 mm inthickness and which is coated with a carbon film at a thickness of lessthan 1 Mm in order to ensure conductivity necessary for an ionimplantation method) coated with 0.2 g of ST-01 was used.

Argon ions were injected at a density of 5 ×10¹⁶ ions/cm². The thusobtained anatase type titanium dioxide sample was subjected to X-rayphotoelectron spectroscopy (XPS), and the area of the peaks assigned tothe 2p-electron of titanium (458.6 eV (Ti2p_(3/2)) and 464.3 eV(Ti2p_(1/2))) as well as that of the peak assigned to the 1s-electron ofthe oxygen bonded to titanium (529.7 eV (O1s)) was obtained. The arearatio (O1s/Ti2p) was found to be 1.76. The area ratio (O1s/Ti2p) of ananatase type titanium dioxide powder not subjected to plasma treatmentwas 2.00.

The sample was left to stand in air for 1 week, and the area ratio wasobtained by performing the same measurement to obtain an area ratio(O1s/Ti2p) of 1.76. The sample was further left for 1 month, but nochange was found on the area ratio (O1s/Ti2p).

Test Example 1

(Decomposition of Acetaldehyde Using Visible Light)

A 0.2-g portion of each of the samples prepared in Examples 1 and 2above was applied to a glass plate (6×6 cm), and the samples (plate)prepared in Example 3 were each placed inside a glass bell jar-typereaction apparatus (1.9 liter in volume) As the light source, a halogenlamp (JDR110V 75WN/S-EK manufactured by Toshiba Lightech Co., Ltd.) wasused together with a glass filter to cut off ultraviolet radiations 420nm or shorter in wavelength. The center luminance of the light sourcewas 100,000 Lx.

After sufficiently evacuating the inside of the system, acetaldehyde wasinjected into the reaction vessel to prepare a reaction gas having thepredetermined concentration (1,000 ppm). After the adsorptionequilibrium of acetaldehyde was achieved, light irradiation wasperformed for a predetermined duration of time. The reaction gas wasanalyzed by using gas chromatography (FID).

The decrease in the concentration of acetaldehyde after the lightirradiation is shown in the Table below. For comparison, a similar testwas performed on a sample not subjected to the plasma treatment, and theresult is given in Table 1 as Comparative

Example 1

TABLE 1 Halogen Lamp (with radiations 420 nm or Time duration of lightshorter in wavelength cut off) irradiation (minutes) Example 1 400 ppm120 Example 2 330 ppm 90 Example 3 520 ppm 60 Comparative Ex-  0 ppm 120ample 1

From the results shown in Table 1, it can be understood that thephotocatalysts according to the present invention, which are eachanatase type titanium dioxide and which contain stable oxygen defects,exhibit high ability in the photodecomposition of acetaldehyde underirradiation of a visible light. Furthermore, the material used inComparative Example 1 shows high adsorption to acetaldehyde, but had nophotodecomposition effect under the irradiation of a visible light.

Example 4

A 5-g portion of a powder of an anatase type titanium dioxide (ST-01,manufactured by Ishihara Sangyo Kaisha) was placed inside a quartzreaction tube 5 cm in inner diameter and 100 cm in length. A RF plasmagenerating apparatus was attached to the quartz reaction tube, and afterevacuating the inside of the reaction tube system to 0.1 Torr by using avacuum pump, a 500-W power electromagnetic wave (at a frequency of 13.56GHz) was irradiated to the powder of anatase type titanium dioxideplaced inside the reaction tube to thereby generate a plasma. Then,gaseous H₂ was introduced inside the system at a flow rate of 500 ml/minto set the pressure inside the system to about 1 Torr. The treatment wascontinued for 30 minutes while stirring the anatase type titaniumdioxide powder placed inside the reaction tube. Further, the wall of thequartz tube was heated to 400° C. by resistance heating using a nichromewire, and was maintained at the same temperature throughout thereaction.

The anatase type titanium dioxide powder thus obtained was subjected toX-ray photoelectron spectroscopy (XPS), and the area of the peaksassigned to the 2p-electron of titanium (458.8 eV (Ti2p_(3/2)) and 464.6eV (Ti2p_(1/2))) as well as that of the peak assigned to the 1s-electronof the oxygen bonded to titanium (531.7 eV (O1s)) was obtained. The arearatio (O1s/Ti2p) was found to be 1.94. The area ratio (O1s/Ti2p) of ananatase type titanium dioxide powder not subjected to plasma treatmentwas 2.00.

The sample was left to stand in air for 1 week, and the area ratio wasobtained by performing the same measurement to obtain an area ratio(O1s/Ti2p) of 1.94. The sample was further left for 1 month, but nochange was found on the area ratio (O1s/Ti2p).

The samples before and after the plasma treatment above were eachsubjected to X-ray diffraction analysis, and, as a result, no change wasobserved between the anatase type titanium dioxide samples obtainedbefore and after the plasma treatment. In FIG. 1 are shown the X-raydiffraction patterns for the sample before plasma treatment (a) and thesample after plasma treatment (b).

Furthermore, the ESR spectra of the samples before and after thetreatment were obtained. The measurements were performed in vacuum (0.1Torr) and at a temperature of 77 K. The measurements were performedunder conditions as follows.

[Basic Parameters]

-   Temperature at measurement: 77K-   Irradiation frequency: 9.2 to 9.4 MHz-   Field: 330 mT±25 mT-   Scanning time: 4 minutes-   Mod.: 0.1 mT-   Gain: 5×10-   Power: 0.1 mW-   Time constant: 0.03 seconds-   Light source: High pressure mercury lamp 500 W-   Filter: L-42    [Sample Preparation]    Vacuum evacuation for 1 hour or longer    [Calculation of g-value]    Mn²⁺ marker (g_(min)=1.981) was taken as the standard, and    calculations were made in accordance with the following equation:    g=g _(mn) ×H _(mn)/(H _(mn) +ΔH)    where, H_(mn) represents the magnetic field of Mn²⁺ marker, and ΔH    represents the change in magnetic field from H_(mn).

The ESR spectra of the sample before subjecting it to plasma treatmentare shown in FIG. 2. Referring to the figure, (a) shows the ESR spectrumin darkness, and (b) shows the ESR spectrum obtained in a state in whichlight was irradiated through a filter (L-42) cutting off the light 420nm or shorter in wavelength from the light emitted from a 500-W highpressure mercury lamp.

In FIG. 3 are shown the ESR spectra of the sample after it was subjectedto plasma treatment. Referring to the figure, (a) shows the ESR spectrumin darkness, (b) shows the ESR spectrum obtained in a state in whichlight was irradiated through a filter (L-42) cutting off the light 420nm or shorter in wavelength from the light emitted from a 500-W highpressure mercury lamp, and (c) shows the ESR spectrum obtained in astate in which light was irradiated without using the filter (L-42).

The ESR spectra in FIG. 2 and FIG. 3 were obtained under the sameconditions. On comparing the results shown in the figures, the catalystaccording to the present invention yields a characteristic signal at a gvalue of 2.003 to 2.004, which was not observed for the startingmaterial. Furthermore, this signal was found to be amplified under theirradiation of light in which the radiations 420 nm or shorter inwavelength are cut off. The catalyst according to the present inventionyielded a signal which increases its intensity under the irradiation ofa visible light 420 nm or longer in wavelength at a g value of from2.003 to 2.004. Moreover, this peak was found to be maintained onre-measuring the sample after allowing it to stand in air for 1 week.Further, no signal assigned to Ti³⁺, which should appear at a g value of1.96, was found for the catalyst obtained in Example 4.

Test Example 2

(Measurement of NOx Oxidation Activity)

A 0.2-g portion of the sample prepared in Example 4 was applied to aglass plate (6×6 cm), and the sample (plate) was placed inside a Pyrexglass reaction vessel (160 mm in inner diameter and 25 mm in thickness).A 300-W xenon lamp was used as the light source, and light wasirradiated as a monochromatic light having a half width value of 20 nmby using a JASCO irradiating apparatus.

Then, a mock contaminated air (containing 1.0 ppm of NO) having ahumidity of 0% RH was continuously supplied to the reaction vessel aboveat a flow rate of 1.5 liter/minute, and the change in concentration ofNO was monitored at the reaction exit. The concentration of NO wasmeasured by means of chemical emission method using ozone. The removalratio for NOx was obtained from the cumulative monitored value for 24hours. The results are shown in Table 2. In Table 2, the resultsobtained on the sample (ST-01) used for the starting material are alsoshown for comparison.

TABLE 2 Wavelength (nm) 360 460 560 NO removal ratio (%) Example 4 28.717.1 4.7 Starting material (ST-01) 28.1 0.2 0

From the results shown in Table 2, it can be understood that thephotocatalyst according to the present invention (the sample obtained inExample 4), which is an anatase type titanium dioxide and which containsstable oxygen defects, exhibits that it effectively removes nitrogenoxides by oxidation under the irradiation of a visible light of at least560 nm or shorter in wavelength. Although not shown in Table 2, thephotocatalyst according to the present invention (the sample obtained inExample 4), which is an anatase type titanium dioxide and which containsstable oxygen defects, possessed an effect of effectively removingnitrogen oxides by oxidation under the irradiation of a visible light ofat least 600 nm or shorter in wavelength.

Test Example 3

(Test on the Reduction of Benzoic Acid)

A 0.02-g portion of the sample prepared in Example 4 was placed inside aPyrex glass reaction vessel (40 ml in volume) together with 25 ml ofbenzoic acid 0.01 mol/l in concentration, and was stirred with amagnetic stirrer. A halogen lamp controlled with a voltage regulator asto yield a power of 70 mW at a wavelength of 500 nm was used as thelight source. The distance between the halogen lamp and the reactioncell was set at 10 cm. A sharp cut filter was placed between the halogenlamp and the reaction cell to cut off ultraviolet radiations. For thereaction, the system was left for 24 hours to establish adsorptionequilibrium, and light was irradiated thereto for 48 hours to initiatethe reaction.

The state before the reaction was compared with that after the reactionby measuring the concentration of benzoic acid; i.e., the absorbance ata wavelength of 228 nm in visible to ultraviolet light absorptionspectrum was measured. No light was allowed during the reaction and themeasurement.

As a result, the decomposition ratio of benzoic acid after 48 hours wasfound to be 20.46%. However, no decomposition of benzoic acid was foundin case of using titanium oxide employed for the starting material underthe conditions above.

Example 5

A 10-g portion of a powder of an anatase type titanium dioxide (ST-01,manufactured by Ishihara Sangyo Kaisha) was placed inside a quartzreaction tube 400 ml in volume. The quartz reaction tube was connectedto a plasma generating apparatus, and after evacuating the inside of thesystem by using a vacuum pump, a 200-W power electromagnetic wave (at afrequency of 2.45 GHz) was irradiated to the powder of anatase typetitanium dioxide placed inside the reaction tube to thereby generate aplasma by using a Tesla coil. Then, gaseous H₂ was introduced inside thesystem at a flow rate of 30 ml/min to set the pressure inside the systemto about 1 Torr. The treatment was continued for 10 minutes whilestirring the anatase type titanium dioxide powder placed inside thereaction tube.

Time duration of 40 minutes was necessary to increase the vacuum degreeinside the plasma treatment system for 1 Torr without introducing a gasand while cutting off the pump evacuation.

The anatase type titanium dioxide powder thus obtained was subjected toX-ray photoelectron spectroscopy (XPS), and the area of the peaksassigned to the 2p-electron of titanium (458.8 eV (Ti2p_(3/2)) and 464.6eV (Ti2p_(1/2))) as well as that of the peak assigned to the 1s-electronof the oxygen bonded to titanium (531.7 ev (O1s)) was obtained. The arearatio (O1s/Ti2p) was found to be 1.92. The area ratio (O1s/Ti2p) of ananatase type titanium dioxide powder not subjected to plasma treatmentwas 2.00.

The sample was left to stand in air for 1 week, and the area ratio wasobtained by performing the same measurement to obtain an area ratio(O1s/Ti2p) of 1.92. The sample was further left for 1 month, but nochange was found on the area ratio (O1s/Ti2p).

The samples before and after the plasma treatment above were eachsubjected to X-ray diffraction analysis, and, as a result, no change wasobserved between the anatase type titanium dioxide samples obtainedbefore and after the plasma treatment.

Furthermore, the ESR spectra of the samples before and after thetreatment were obtained. The measurements were performed in the samemanner as in Example 4. As a result, similar to the case in Example 4, asignal was observed at a g value of 2.003 to 2.004 for the catalyst (ananatase type titanium dioxide subjected to plasma treatment) obtained inExample 5. Moreover, this peak was found to be maintained onre-measuring the sample after allowing it to stand in air for 1 week.Further, no signal assigned to Ti³, which should appear at a g value of1.96, was found for the catalyst obtained in Example 5.

Example 6

A 4-g portion of a powder of an anatase type titanium dioxide (ST-01,manufactured by Ishihara Sangyo Kaisha) was placed inside a quartzreaction tube 200 ml in volume. An electric heating wire heater wasattached to the quartz reaction tube, and after evacuating the inside ofthe system to a vacuum degree of 0.1 Torr or lower by using a vacuumpump, the entire reaction tube was heated to a temperature of 400° C. bythe heater. Heating was continued to maintain the temperature to 400° C.for an hour.

During the treatment, the evacuation was continued by using a vacuumpump to maintain the vacuum degree to 0.1 Torr or lower. Thus, a darkbrown colored powder was obtained after 1 hour.

The anatase type titanium dioxide powder thus obtained was subjected toX-ray photoelectron spectroscopy, and the area of the peaks assigned tothe 2p-electron of titanium (459.5 eV (Ti2p_(3/2)) and 465.4eV(Ti2p_(1/2))) as well as that of the peak as signed to the 1s-electronof the oxygen bonded to titanium (530.0 ev (O1s)) was obtained. The arearatio (O1s/Ti2p) was found to be 1.92. The area ratio (O1s/Ti2p) of ananatase type titanium dioxide powder not subjected to plasma treatmentwas 2.00.

The sample was left to stand in air for 1 week, and the area ratio wasobtained by performing the same measurement to b obtain an area ratio(O1s/Ti2p) of 1.92. The sample was further left for 1 month, but nochange was found on the area ratio (O1s/Ti2p).

The samples before and after the plasma treatment above were eachsubjected to X-ray diffraction analysis, and, as a result, no change wasobserved between the anatase type titanium dioxide samples obtainedbefore and after the plasma treatment.

Test Example 4

(Measurement of NOx Oxidation Activity)

The NOx oxidation activity of the samples prepared in Example 6 wasmeasured under the same conditions as those used in Test Example 2. Theresults are given in Table 3. It can be understood that the sampleobtained in Example 6 yields an activity slightly lower (particularly inthe short wavelength regions) than that of the sample obtained inExample 4, however, the activity was observed to a wavelength in thevicinity of 600 nm.

TABLE 3 Wavelength (nm) 360 460 560 NO removal ratio (%) Example 6 18.316.2 4.7 Starting material (ST-01) 28.1 0.2 0

Test Example 5

(Test on the Reduction of Benzoic Acid)

A photodecomposition test of benzoic acid was performed by using thesample prepared in Example 6 under the same conditions as those used inTest Example 3.

As a result, the decomposition ratio of benzoic acid after 48 hours wasfound to be 15.42%. However, no decomposition of benzoic acid was foundin case of using titanium oxide employed for the starting material underthe conditions above.

Example 7

A 5-g portion of a powder of an anatase type titanium dioxide (ST-01,manufactured by Ishihara Sangyo Kaisha) was placed inside a quartzreaction tube 5 cm in inner diameter and 100 cm in length. A RF plasmagenerating apparatus was attached to the quartz reaction tube, and afterevacuating the inside of the reaction tube system to 0.05 Torr by usinga vacuum pump, a 500-W power electromagnetic wave (at a frequency of13.56 GHz) was irradiated to the powder of anatase type titanium dioxideplaced inside the reaction tube to thereby generate a plasma. Then,gaseous H₂ was introduced inside the system at a flow rate of 500 ml/minto set the pressure inside the system to about 1 Torr. The treatment wascontinued for 30 minutes while stirring the anatase type titaniumdioxide powder placed inside the reaction tube. Further, the wall of thequartz tube was heated to 400° C. by resistance heating using a nichromewire, and was maintained at the same temperature throughout thereaction.

The anatase type titanium dioxide powder thus obtained was subjected toX-ray photoelectron spectroscopy (XPS), and the area of the peaksassigned to the 2p-electron of titanium (458.8 eV (Ti2p_(3/2)) and 464.6ev (Ti2p_(1/2))) as well as that of the peak assigned to the 1s-electronof the oxygen bonded to titanium (531.7 eV (O1s)) was obtained. The arearatio (O1s/Ti2p) was found to be 1.51. The area ratio (O1s/Ti2p) of ananatase type titanium dioxide powder not subjected to plasma treatmentwas 2.00.

The sample was left to stand in air for 1 week, and the area ratio wasobtained by performing the same measurement to obtain an area ratio(O1s/Ti2p) of 1.51. The sample was further left for 1 month, but nochange was found on the area ratio (O1s/Ti2p).

Example 8

Plasma Treatment of Zirconia

A 2-g portion of ZrO₂ manufactured by Kishida Kagaku K. K. was placedinside a 280-ml volume quartz reaction tube. The quartz reaction tubewas connected to a plasma generating apparatus, and after evacuating theinside of the system by using a vacuum pump, a 400-W powerelectromagnetic wave (at a frequency of 2.45 GHz) was irradiated to thezirconia powder placed inside the reaction tube to thereby generate aplasma by using a Tesla coil. Then, gaseous H₂ was introduced inside thesystem at a flow rate of 30 ml/min to set the pressure inside the systemto about 1 Torr. The treatment was continued for 30 minutes whilestirring the zirconia powder placed inside the reaction tube.

The thus obtained zirconium oxide powder was subjected to x-rayphotoelectron spectroscopy, and the area of the peaks assigned to the3d-electron of zirconium (182 to 183 eV (Zr3d_(5/2)) and 184 to 185 eV(Zr3d_(3/2))) as well as that of the peak assigned to the 1s-electron ofthe oxygen bonded to zirconium (530 eV (O1s)) was obtained. The arearatio (O1s/Zr3d) was found to be 1.98. The area ratio (O1s/Zr3d) ofzirconium oxide powder not subjected to plasma treatment was 2.01.

The sample was left to stand in air for 1 week, and the area ratio wasobtained by performing the same measurement to obtain an area ratio(O1s/Zr3d) of 1.98. The sample was further left for 1 month, but nochange was found on the area ratio (O1s/Zr3d).

Test Example 6

A 0.2-g portion of the samples prepared in Example 8 was placed inside aglass bell jar-type reaction apparatus (1.9 liter in volume). As thelight source, a halogen lamp (JDR100V 75WN/S-EK manufactured by ToshibaLightech Co., Ltd.) was used together with a glass filter to cut offultraviolet radiations 390 nm or shorter in wavelength. The centerluminance of the light source was 100,000 Lx.

After sufficiently evacuating the inside of the system, acetaldehyde wasinjected into the reaction vessel to prepare a reaction gas having thepredetermined concentration (500 ppm) After the adsorption equilibriumof acetaldehyde was achieved, light irradiation was performed for apredetermined duration of time. The reaction gas was analyzed by usinggas chromatography (FID). The concentration of acetaldehyde 120 minutesafter initiating the light irradiation is shown in the Table 4 below.For comparison, the acetaldehyde was measured 120 minutes after theinitiation of the light irradiation for a zirconia starting material notsubjected to plasma treatment, and the results are shown in Table 4.

Test Example 7

A 0.2-g portion of the sample prepared in Example 8 was placed inside aglass bell jar-type reaction apparatus (1.9 liter in volume). As thelight source, a black lamp (H110BL, manufactured by Iwasaki Denki Co.,Ltd.) having a UV intensity of 1.8 mW/cm² was used to irradiateultraviolet radiations in the UVa region.

After sufficiently evacuating the inside of the system, acetaldehyde wasinjected into the reaction vessel to prepare a reaction gas having thepredetermined concentration (500 ppm). After the adsorption equilibriumof acetaldehyde was achieved, light irradiation was performed for apredetermined duration of time. The reaction gas was analyzed by usinggas chromatography (FID). The concentration of acetaldehyde 120 minutesafter initiating the light irradiation is shown in the Table 4 below.For comparison, the acetaldehyde was measured 120 minutes after theinitiation of the light irradiation for a zirconia starting material notsubjected to plasma treatment, and the results are shown in Table 4.

TABLE 4 Halogen lamp (radiations 390 nm or shorter in Black wavelengthcut off) light Example 8 268 ppm 250 ppm Comparative Example 2 (zirconiastarting material) 499 ppm 489 ppm

From the results shown in Table 4, it can be understood that thehydrogen-plasma treated zirconia prepared in accordance with theproduction method of the present invention exhibits high ability in thephotodecomposition of acetaldehyde under irradiation of UVa and visiblelight; thus, it functions as a photocatalyst under the irradiation of avisible light. Furthermore, zirconia used as the starting material inComparative Example 2 had no photodecomposition effect on acetaldehydeunder the irradiation of a visible light or an ultraviolet radiation.

Example 9

Plasma Treatment of Rutile Type TiO₂

A 2-g portion of a rutile type TiO₂ (MT-500B) manufactured by Teika Inc.was placed inside a 280-ml volume quartz reaction tube. The quartzreaction tube was connected to a plasma generating apparatus, and afterevacuating the inside of the system by using a vacuum pump, a 400-wpower electromagnetic wave (at a frequency of 2.45 GHz) was irradiatedto the rutile type titanium oxide powder placed inside the reaction tubeto thereby generate a plasma by using a Tesla coil. Then, gaseous H₂ wasintroduced inside the system at a flow rate of 30 ml/min to set thepressure inside the system to about 1 Torr. The treatment was continuedfor 30 minutes while stirring the rutile type titanium oxide powderplaced inside the reaction tube. As a result, a bluish pale gray powderwas obtained. The samples before and after the plasma treatment abovewere each subjected to X-ray diffraction analysis, and, as a result, nochange was observed on the rutile type titanium dioxide obtained beforeand after the plasma treatment.

The thus obtained rutile type titanium dioxide powder was subjected toX-ray photoelectron spectroscopy, and the area of the peaks assigned tothe 2p-electron of titanium (458.6 eV (Ti2p_(3/2)) and 464.2eV(Ti2p_(1/2))) as well as that of the peak as signed to the 1s-electronof the oxygen bonded to titanium (529.8 eV (O1s)) was obtained. The arearatio (O1s/Ti2p) was found to be 1.74. The area ratio (O1s/Ti2p) ofrutile type titanium dioxide powder not subjected to plasma treatmentwas 2.01. Further, the sample was left to stand in air for 1 week, andthe area ratio was obtained by performing the same measurement to obtainan area ratio (O1s/Ti2p) of 1.74. The sample was further left for 1month, but no change was found on the area ratio (O1s/Ti2p).

A 0.2-g portion of the sample prepared above was placed inside a glassbell jar-type reaction apparatus (1.9 liter in volume). As the lightsource, a halogen lamp (JDR110V 75WN/S-EK manufactured by ToshibaLightech Co., Ltd.) was used together with a glass filter to cut offultraviolet radiations 390 nm or shorter in wavelength. The centerluminance of the light source was 100,000 Lx.

After sufficiently evacuating the inside of the system, acetaldehyde wasinjected into the reaction vessel to prepare a reaction gas having thepredetermined concentration (500 ppm). After the adsorption equilibriumof acetaldehyde was achieved, light irradiation was performed for apredetermined duration of time. The reaction gas was analyzed by usinggas chromatography (FID). The concentration of acetaldehyde 120 minutesafter initiating the light irradiation is shown in the Table 5. Forcomparison, the acetaldehyde was measured 50 minutes after theinitiation of the light irradiation for a rutile type TiO₂ not subjectedto plasma treatment, and the results are shown in Table 5.

TABLE 5 Halogen lamp (radiations 390 nm or shorter in wavelength cutoff) Example 9 264 ppm Comparative Example 3 302 ppm (rutile typetitanium oxide starting material)

From the results shown in Table 5, it can be understood that thehydrogen-plasma treated rutile type titanium oxide prepared inaccordance with the production method of the present invention exhibitshigh ability in the photodecomposition of acetaldehyde under irradiationof visible light; thus, it functions as a photocatalyst under theirradiation of a visible light. On the other hand, the rutile typetitanium oxide used as the starting material in Comparative Example 3showed photodecomposition properties on acetaldehyde under theirradiation of a visible light, but the effect was weaker as comparedwith the sample obtained in Example 3.

Example 10

Plasma Treatment of Hafnium Oxide

A 2-g portion of hafnium oxide (HfO₂ of 99.8% purity, manufactured byFluka Inc.) was placed inside a 200-ml volume quartz reaction tube. Thequartz reaction tube was connected to a plasma generating apparatus, andafter evacuating the inside of the system by using a vacuum pump, a400-W power electromagnetic wave (at a frequency of 2.45 GHz) wasirradiated to the hafnium oxide powder placed inside the reaction tubeto thereby generate a plasma by using a Tesla coil. Then, gaseous H₂ wasintroduced inside the system while controlling the flow rate to 30ml/min by using a mass flow meter, such that the pressure inside thesystem was set to about 1 Torr. The treatment was continued for 1 hourwhile rotating the quartz reaction tube and stirring the hafnium oxidepowder placed inside the reaction tube. As a result, a powder having agray surface was obtained.

The thus obtained hafnium oxide powder was subjected to X-rayphotoelectron spectroscopy, and the area of the peaks assigned to the4f-electron of hafnium (16 to 17 eV (Hf4f)) as well as that of the peakassigned to the 1s-electron of the oxygen bonded to hafnium (530 eV(O1s)) was measured. The area ratio (O1s/Hf4f) was found to be 2.15. Thearea ratio (O1s/Hf4f) of hafnium oxide powder not subjected to plasmatreatment was 2.20.

The sample was left to stand in air for 1 week, and the area ratio wasobtained by performing the same measurement to obtain an area ratio(O1s/Hf4f) of 2.15. The sample was further left for 1 month, but nochange was found on the area ratio (O1s/Hf4f).

A 0.4-g portion of the sample prepared above was dispersed in methanol,and was applied to a glass plate (6×6 cm). The glass plate was then setinside a glass bell jar-type reaction apparatus (1.9 liter in volume).As the light source, a halogen lamp (JDR110V 75WN/S-EK manufactured byToshiba Lightech Co., Ltd.) was used together with a glass filter to cutoff ultraviolet radiations 420 nm or shorter in wavelength.

After sufficiently evacuating the inside of the system, acetaldehyde wasinjected into the reaction vessel to prepare a reaction gas having thepredetermined concentration (500 ppm) After the adsorption equilibriumof acetaldehyde was achieved, light irradiation was initiated. Thereaction gas was analyzed by using gas chromatography (FID). Theconcentration of acetaldehyde at 90 minutes after the initiation of theirradiation was found to be 420 ppm. Separately, a 0.4-g portion of anuntreated hafnium oxide powder was similarly coated to obtain a sample,and the sample was subjected to the test. However, no change inacetaldehyde concentration was found before and after the irradiation oflight. From the above results, it can be understood that the hydrogenplasma treated hafnium oxide prepared in accordance with the productionmethod of the present invention exhibits photodecomposition propertieson acetaldehyde under the irradiation of a visible light, and that itfunctions as a photocatalyst using visible light.

Example 11

Plasma Treatment of Strontium Titanate

A 2-g portion of strontium titanate (SrTiO₂ of 99% purity and consistingof particles 5 μm or less in diameter, manufactured by Aldrich ChemicalCompany) was placed inside a 200-ml volume quartz reaction tube. Thequartz reaction tube was connected to a plasma generating apparatus, andafter evacuating the inside of the system by using a vacuum pump, a400-W power electromagnetic wave (at a frequency of 2.45 GHz) wasirradiated to the strontium titanate powder placed inside the reactiontube to thereby generate a plasma by using a Tesla coil. Then, gaseousH₂ was introduced inside the system while controlling the flow rate to30 ml/min by using a mass flow meter, such that the pressure inside thesystem was set to about 1 Torr. The treatment was continued for 1 hourwhile rotating the quartz reaction tube and stirring the strontiumtitanate powder placed inside the reaction tube. As a result, a powderhaving a gray surface was obtained.

A 0.2-g portion of the sample prepared above was dispersed in methanol,and was applied to a glass plate (6×6 cm). The glass plate was then setinside a glass bell jar-type reaction apparatus (1.9 liter in volume).As the light source, a halogen lamp (JDR110V 75WN/S-EK manufactured byToshiba Lightech Co., Ltd.) was used together with a glass filter tocutoff ultraviolet radiations 420 nm or shorter in wavelength.

After sufficiently evacuating the inside of the system, acetaldehyde wasinjected into the reaction vessel to prepare a reaction gas having thepredetermined concentration (500 ppm) After the adsorption equilibriumof acetaldehyde was achieved, light irradiation was initiated. Thereaction gas was analyzed by using gas chromatography (FID). Theconcentration of acetaldehyde at 60 minutes after the initiation of theirradiation was found to be 450 ppm. Separately, a 0.4-g portion of anuntreated strontium titanate powder was similarly coated to obtain asample, and the sample was subjected to the test. However, no change inacetaldehyde concentration was found before and after the irradiation oflight. From the above results, it can be understood that the hydrogenplasma treated strontium titanate prepared in accordance with theproduction method of the present invention exhibits photodecompositionproperties on acetaldehyde under the irradiation of a visible light, andthat it functions as a photocatalyst using visible light.

Example 12

Wet Solar Cell

The catalyst (powder) according to the present invention prepared inExample 1 was mixed with polyethylene glycol and acetone, and wasapplied to a transparent electrode (ITO). After coating, bakingtreatment was applied thereto at ca. 300° C. for time duration of 1hour. The electrode thus obtained was immersed into a methanol solutioncontaining a commercially available ruthenium complex (8RuL2 (NCS)2,L=4,4′-dicarboxy-2,2′-bispyridine). Then, a drop of aqueous solutioncontaining 0.1 M of potassium iodide was applied to the resultingelectrode. A transparent electrode (ITO) was superposed thereon toprovide a counter electrode, and the surroundings were fixed with aresin to obtain a wet solar cell. A light emitted from a halogen lamp(JDR110V 75WN/S-EK manufactured by Toshiba Lightech Co., Ltd.) wasirradiated to the battery through a glass filter which cuts offultraviolet radiations 420 nm or shorter in wavelength. As a result, thegeneration of a photoelectric current was observed.

Similarly, a wet solar cell was prepared in the same manner as above byusing the catalyst (powder) according to the present invention preparedin Example 4. A light emitted from a halogen lamp (JDR110V 75WN/S-EKmanufactured by Toshiba Lightech Co., Ltd.) was irradiated to thebattery through a glass filter which cuts off ultraviolet radiations 420nm or shorter in wavelength. As a result, the generation of aphotoelectric current was similarly observed.

Example 13

Wet Solar Cell

Wet solar cells were each prepared in the same manner as in Example 12by using the samples obtained in Examples 1 and 4, except for using apolyaniline thin film electrode in the place of transparent electrode(ITO). A light emitted from a halogen lamp (JDR110V 75WN/S-EKmanufactured by Toshiba Lightech Co., Ltd.) was irradiated to each ofthe cells through a glass filter which cuts off ultraviolet radiations420 nm or shorter in wavelength. As a result, the generation of aphotoelectric current was similarly observed.

Example 14

Test on Water Decomposition using Visible Light

A 0.3-g portion of the photocatalyst prepared in Example 1, water (pH7,30 ml in volume), and a magnetic stirrer were set inside a reactionvessel, and the entire system was connected to a vacuum evacuation line(500 ml in volume). A 500-W xenon lamp was used as the light source, anda glass filter to cut off ultraviolet radiations 420 nm or shorter inwavelength was incorporated.

After sufficiently evacuating the inside of the system, the irradiationof light was initiated. Gaseous hydrogen generated in the system wascollected every 5 hours, and was analyzed by gas chromatography (TCD).Gaseous hydrogen was generated at a rate of 0.02 μmol/h.

Example 15

Test on the Reduction of Carbon Dioxide Using Visible Light

A 0.3-g portion of the photocatalyst prepared in Example 1 was dispersedin methanol, and after coating the resulting dispersion to a glass plate(6 cm×6 cm), the sample was heated at 300° C. for 1 hour to obtain asample which is less apt to cause powder desorption. After placing theglass plate coated with the photocatalyst inside a 1-liter volumereaction vessel, the vessel was connected to a vacuum evacuation line(500 ml in volume). After evacuating the system, gaseous carbon dioxide(500 ppm) passed through a water vapor phase was injected into thereaction vessel. A 500-W xenon lamp was used as the light source, and aglass filter to cut off ultraviolet radiations 420 nm or shorter inwavelength was incorporated. Gas chromatography (TCD) was used for theanalysis of the gas evolved.

Visible and infrared radiations were irradiated to the reaction vessel,and the mixed gas inside the reaction vessel was analyzed every time ofirradiation, and was found that methanol generated at a rate of 2μmol/h.

As described above, the present invention provides photocatalysts havingactivity under the irradiation of visible light, and by using thecatalysts, substances such as acetaldehyde, NOx, benzoic acid, etc., canbe photo decomposed.

The material according to the present invention can be applied tovarious fields using the activity under visible light.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

1. A catalyst having activity under an irradiation of visible light in awavelength region from about 400 to 600 nm, comprising titanium dioxidehaving stable oxygen defects and exhibiting NOx oxidation activity underthe irradiation of a visible light at least in the wavelength region offrom about 400 to 600 nm; and said titanium dioxide further having apeak area ratio (O1s/Ti2p) of a peak area obtained by X-rayphotoelectron spectroscopy assigned to the 1s electrons of oxygen (O1s)participating in the bonds with titanium to a peak area obtained byX-ray photoelectron spectroscopy assigned to the 2p electrons oftitanium (Ti2p) of 1.99 or lower.
 2. The catalyst according to claim 1,wherein said titanium dioxide component comprises titanium dioxide of ananatase type or a rutile type.
 3. The catalyst according to claim 1,wherein the titanium dioxide has a primary particle size of 10 nm orless in diameter.
 4. The catalyst according to claim 1, comprisingtitanium dioxide that is characterized by an X-ray diffraction (XRD)pattern that is substantially free from patterns other than patternsassigned to anatase type titanium dioxide.
 5. The catalyst according toclaim 1, wherein said catalyst is in a substantially granular,thin-film, or sheet shape.
 6. The catalyst of claim 1, wherein saidcatalyst material has been provided on the surface of a base materialsubstrate.
 7. The catalyst article according to claim 6, wherein saidbase material is an exterior wall of a building, an exterior plane of aroof or a ceiling, an outer plane or an inner plane of a window glass,an interior wall of a room, a floor or a ceiling, a blind, a curtain, aprotective wall of highway roads, an inner wall inside a tunnel, anouter plane or a reflective plane of an illuminating light, an interiorsurface of a vehicle, or a plane of a mirror.
 8. A catalyst havingactivity under an irradiation of visible light, said catalyst comprisingtitanium dioxide having stable oxygen defects and a peak area ratio(O1s/Ti2p) of a peak area obtained by X-ray photoelectron spectroscopyassigned to the 1Is electrons of oxygen (O1s) participating in the bondswith titanium to a peak area obtained by X-ray photoelectronspectroscopy assigned to the 2p electrons of titanium (Ti2p) of 1.99 orlower.
 9. The catalyst according to claim 8, wherein said peak arearatio (O1s/Ti2p) is in a range of from 1.5 to 1.95.
 10. The catalystaccording to claim 8, wherein said peak area ratio (O1s/Ti2p) remainssubstantially constant for time durations of 1 week or longer.
 11. Thecatalyst according to claim 8, wherein said activity under theirradiation of visible light is an oxidation activity or a reductionactivity.
 12. The catalyst according to claim 8, wherein said activityunder the irradiation of visible light is a decomposition activity forinorganic and organic substances, or a bactericidal activity.
 13. Amethod for producing a catalyst comprising titanium dioxide havingstable oxygen defects and a ratio of a peak area obtained by X-rayphotoelectron spectroscopy assigned to the Is electrons of oxygenparticipating in the bonds with titanium to a peak area obtained byX-ray photoelectron spectroscopy assigned to the 2p electrons oftitanium (O1s/Ti2p) of 1.99 or lower and having activity under anirradiation of a visible light, said method comprising treating thetitanium dioxide with hydrogen plasma, characterized by performing saidtreatment in a state substantially free from an intrusion of air into atreatment system.
 14. The method for producing a catalyst according toclaim 13, wherein said treatment is performed in a tightly sealed systemand said state substantially free from the intrusion of air into thetreatment system is a state in which a vacuum degree inside the tightlysealed system takes at least 10 minutes to make a change of 1 Torr. 15.The method for producing a catalyst according to claim 13, wherein saidoxide semiconductor is selected from the group consisting of titaniumdioxide, zirconium oxide, hafnium oxide, strontium titanate, a titaniumoxide-zirconium oxide based complex oxide, and a silicon oxide-titaniumoxide based complex oxide.
 16. The method for producing a catalystaccording to claim 13, wherein said oxide semiconductor is an anatasetype titanium dioxide.
 17. A catalyst produced by the method of claim 13and having activity under the irradiation of a visible light.
 18. Thecatalyst according to claim 17, wherein said oxide semiconductor istitanium dioxide, zirconium oxide, hafnium oxide, strontium titanate, atitanium oxide-zirconium oxide based complex oxide, or a siliconoxide-titanium oxide based complex oxide.
 19. A method for producing acatalyst comprising titanium dioxide having stable oxygen defects and aratio of a peak area obtained by X-ray photoelectron spectroscopyassigned to the 1s electrons of oxygen participating in the bonds withtitanium to a peak area obtained by X-ray photoelectron spectroscopyassigned to the 2p electrons of titanium (O1s/Ti2p) of 1.99 or lower andhaving activity under an irradiation of a visible light, said methodcomprising treating the titanium dioxide with a plasma of rare gas, andperforming said treatment in a state substantially free from anintrusion of air into a treatment system.
 20. The method for producing acatalyst according to claim 19, wherein said state substantially freefrom the intrusion of air into the treatment system is a state in whicha vacuum degree inside a tightly sealed system takes at least 10 minutesto make a change of 1 Torr.
 21. The method for producing a catalystaccording to claim 19, wherein said oxide semiconductor is selected fromthe group consisting of titanium dioxide, zirconium oxide, hafniumoxide, strontium titanate, a titanium oxide-zirconium oxide basedcomplex oxide, and a silicon oxide-titanium oxide based complex oxide.22. The method for producing a catalyst according to claim 19, whereinsaid oxide semiconductor is an anatase type titanium dioxide.
 23. Acatalyst produced by the method of claim 19 and having activity underthe irradiation of a visible light.
 24. The catalyst according to claim23, wherein said oxide semiconductor is titanium dioxide, zirconiumoxide, hafnium oxide, strontium titanate, a titanium oxide-zirconiumoxide based complex oxide, or a silicon oxide-titanium oxide basedcomplex oxide.
 25. A method for producing a catalyst comprising titaniumdioxide having stable oxygen defects and a ratio of a peak area obtainedby X-ray photoelectron spectroscopy assigned to the 1s electrons ofoxygen participating in the bonds with titanium to a peak area obtainedby X-ray photoelectron spectroscopy assigned to the 2p electrons oftitanium (O1s/Ti2p) of 1.99 or lower and having activity under anirradiation of visible light, comprising the step of introducing ions ofa rare gas on at least a portion of the surface of the titanium dioxideby means of ion implantation.